A kamado is the Japanese term for a traditional cooking stove fueled by wood or charcoal. In its more modern sense, the term kamado has come to denote a wood-fired or charcoal-fired cooking vessel typically made from ceramic, clay, terracotta or crushed lava rock to create a grill that can withstand temperatures in excess of 750° F. without cracking from extreme heat or temperature fluctuations. Of course, modern kamado-style grills may be formed from any suitable refractory materials, included those listed above as well as various metals or metal alloys. Kamado-style grills/smokers are typically thought of as having a circular or oval cross-section in the horizontal plane, though square, rectangular or other shaped kamado-style cookers are certainly within the contemplation of the present invention.
A kamado typically comprises an egg-shaped body with a hinged, domed top, the body being made of relatively thick ceramic or other refractory material. Kamado style grills usually have a hinged top because the ceramic top is very heavy and would be difficult to handle if it were not attached. Both the base and the top have one or more adjustable vents, chimneys or air control dampers to provide the user with a means for controlling the flow of air through the cooking chamber.
Most kamado grills have a high gloss ceramic glaze or are enameled to retain a glossy exterior finish over time despite exposure to the elements. The body, both the lower kettle or grill base and the domed top or grill dome comprise relatively thick ceramic or clay walls to provide a rigid structure as well as retain the heat within the body. Ceramics are a preferred refractory material because they retain the heat within the cooking chamber and do not conduct heat very well, so, while not a perfect insulator, the ceramic walls do provide a significant measure of insulation in that they do not readily allow the passage of heat.
The construction materials and good air control gives these grills excellent insulation, high heating efficiency, and the ability to hold very high temperatures without significant heat loss, making them especially suited for a wide range of grilling, roasting, baking and smoking.
The cooking chamber of a kamado grill, i.e., the enclosed portion containing the heating fuel and the cooking surface, is generally ovoid in shape with a circular or oval horizontal cross-section. The cooking chamber is usually heated by a combustible fuel, such as charcoal or wood, placed in the bottom of the cooking chamber formed within the grill base. In order to hold the fuel and to better shape the fire, a firebox is disposed within the base. The firebox helps to protect the grill base by separating the burning fuel from the walls of the grill base, thereby reducing the wear of expansion and contraction of the walls which can, over time, lead to cracking of the refractory materials.
The firebox typically supports a grilling surface. In some embodiments, the firebox supports a separate fire ring which, in turn, supports the grilling surface. In other embodiments, the firebox may also support a rack or rack holder to support the grilling surface.
The firebox typically also supports a lower grate above the floor of the grill base. The lower grate divides the firebox into an upper chamber and a lower chamber. The lower grate serves to support the fuel, allow air to flow from below the burning fuel, and allow ash to fall through the grate to the floor or bottom of the grill base.
Because the firebox contains the fuel and ashen remains of the spent fuel, fireboxes typically have an opening at the bottom through which ashes may be removed. The firebox is preferably positioned such that the firebox ash openings are aligned with a corresponding ash removal opening in the grill base to provide a direct path for ash to be removed from the firebox and grill base. Firebox ash openings are typically covered, such as with a door or vent that may be opened or closed as necessary. Of course, any vents provided are preferably themselves adjustable so that air flow through the cooking chamber may be more precisely regulated.
Fireboxes and fire rings are also manufactured from refractory material, typically clay or ceramics and as such, can be very heavy, thick and massive. A typical ceramic firebox weighs between about 20 kg to about 40 kg, depending on the size of the grill. The thickness of the firebox side wall and base may range between about 30 mm to about 40 mm. For example, Applicant in planning to introduce the inventive firebox of the present invention in two different sizes of kamado grills. The assembled firebox of the smaller kamado grill will weigh about 24 kg and the thickness of the base and side wall members will be between about 33 mm to about 35 mm. The assembled firebox of the larger kamado grill will weigh about 36 kg, and the thickness of the base and side wall members will be between about 35 mm to about 36 mm.
As used in this disclosure, “refractory materials,” also referred to simply as “refractories,” are heat-resistant materials that retains their strength at high temperatures. Refractories provide containment of substances at high temperatures. ASTM C71 defines refractories as “non-metallic materials having those chemical and physical properties that make them applicable for structures, or as components of systems, that are exposed to environments above 1,000° F. (811 K; 538° C.).”
Refractories, such as clay and, more preferably, ceramic materials, are especially suited to use as the material from which a firebox is made due to their relatively low thermal coefficient of expansion. Thermal expansion is the tendency of a material to change in size, shape, area or volume in response to a change in temperature. The coefficient of thermal expansion describes how the size of an object changes with a change in temperature. Having a lower or smaller thermal coefficient of thermal expansion means a material will expand (or contract) less as the temperature rises (or falls). Ceramics do not conduct heat very well, and have a low thermal coefficient of expansion. Comparatively, copper conducts heat very quickly, as do most metals, which also have higher thermal coefficients of expansion than do ceramics. Accordingly, ceramics can absorb a great quantity of heat without cracking. But for any given piece of material, if heat is applied non-uniformly, such as around a pile of burning wood, coal, charcoal or other fuel, the piece becomes more susceptible to cracking due to different areas of the piece expanding and contracting at different rates. In other words, if a refractory material experiences a homogeneous temperature throughout, it will expand and contract homogeneously. But when heat is applied across a single piece of refractory material in a non-homogeneous manner, different parts or areas of the material will experience different temperatures and experience temperature changes at different rates. Thus a refractory material that experiences temperature changes non-homogeneously will be more succeptable to cracking as compared to a piece that experiences substantially homogeneous temperatures throughout.
Despite being manufactured out of refractories, the fireboxes of kamado-style grills experience the highest rate of failure of any component of a grill. Because its purpose is to shape the fire and protect the walls of the grill base, the firebox bears the brunt of the exposure to extreme heat and rapid temperature change. Refractories, such as ceramic materials do not expand much when exposed to higher temperatures, but they do still expand, even if it is less than another material might. The constant, relatively rapid expansion and contraction coupled with non-uniformity of temperatures within the firebox, lead to expansion and contraction at different areas of the firebox at different rates. For example, because the fuel is held within the upper chamber of the firebox above the lower grate, and only ash from spent fuel or small burning bits of fuel or meat fall below the grate into the lower chamber of the firebox, the temperatures experienced by the material of the upper chamber are much higher than the temperatures experienced by the material out of which the lower chamber is constructed. And even within the upper chamber, the temperatures experienced and the rates of temperature change experienced by different portions of the bowl of the upper chamber vary widely.
The effect of all of these different expansions and contractions of the firebox, despite being minimized by the nature of the refractory materials, tend to take a toll over time, resulting in cracking of the firebox. Obviously, the firebox cracking is much preferable to cracks developing within the walls of the grill itself, but the need exists for an improved firebox which is less susceptible to cracking and failure.
Additionally, because fireboxes have the highest frequency of failure due to cracking, significant costs are incurred in replacing them, both on the consumer side and by manufacturers when the firebox remains under warranty. Because of the size and weight of existing fireboxes, simply shipping such large pieces incurs significant expense. In addition to a firebox that suffers a reduction in the frequency of breakage, there is a need to reduce the cost and expense of replacement fireboxes.
It is known to manufacture “advanced ceramics,” also known as “fine ceramics,” “technical ceramics” or “engineered ceramics” designed to have lower coefficients of thermal expansion. Similarly, composite ceramics with additives or elements have been used to try to improve the material's ability to withstand cracking due to heating and cooling. Such elements include mullite, spodumene and cordierite. Ceramics may also be fiber reinforced. These techniques may be employed in the manufacture of grills, but forming grills and grill components with such advanced or composite ceramics increases cost.
Thus there remains a need for an improved firebox design that will decrease the rate of firebox failure due to cracking of the walls of the firebox. Similarly, there remains a need for an improved firebox design that can reduce reliance on advanced or composite ceramics as the sole means for avoiding cracking and breakage and allows less expensive alternative refractories to be utilized while still achieving an improved performance against cracking. There also remains a need to reduce the cost of having to replace fireboxes that fail.